It is a widely held belief that regular exercise decreases the risk of acquiring infections; whereas excessive training increases this risk. The ability of exercise to alter the inflammatory response in healthy children has been recently demonstrated. Information in children with cystic fibrosis (CF) does not exist regarding the impact of exercise on the immune system. We propose to perform a comprehensive evaluation of the immune system before and after exercise in children with CF in order to determine: 1) If they are different at baseline from a group of healthy age-matched children; and 2) if the changes in immune parameters following exercise in the CF group are consistent with the changes observed in the healthy group. The effects of exercise on the immune system will be measured by analysis of white blood cells and subsets including natural killer cells. If it is determined that a child with CF can increase their natural defense systems in response to exercise, then one could propose chronic exercise training as a therapeutic modality for enhancing the infection fighting ability. This study will provide a better understanding of the physiologic response to exercise in children with CF so that rationally based therapeutic strategies can be designed.